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what is the best engine for a 15.4 ton spacecraft?


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Answer: That depends entirely on what your doing with the craft.  Is it going to operate in atmosphere?  Is it going to strictly fly in space?  Is it an orbital transfer vehicle?  Is it an all in one?

 

There are several different engines that qualify as the "best" for a craft of that size.  But each is the "best" at a different task.  If you give some more info on what your using it for, one of us can narrow the engine choices down for you. :)

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1 hour ago, JWOC said:

quick question,

what is the best engine for a 15.4 ton spacecraft?

JWOC

What is the 15.4 tons doing?  Is that a payload that's already gotten to its destination via stages you've already dropped, or is it the payload you need to get to orbit?

Oh, and welcome to the forums.

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the capsule and service module is 15.4 tons by itself. i plan on using it to do missions in low to high kerbin orbit, duna and the moon. and other planets.  it will strictly fly in space. @geschosskopf thanks. 

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+1 to the previous posters' advice.

Welcome to the forums!  A helpful tip for posting here:  generally the best way to get short, helpful answers (instead of a wall of text such as the one I'm about to throw at you) is, don't ask "what should I do" questions.  They're open-ended and will get long, confusing answers.  Instead, go ahead and try something, and if you have a problem with it, post a screenshot and ask a "what's wrong with this?" question.  That's a much more specific question and will get nice, short, to-the-point answers.  :)

Anyway, some general advice for picking engines for the launchpad (if this is what you're asking):  It's good to have a TWR on the launchpad somewhere in the 1.3 to 1.5 range.

So, for picking an engine:  Take the mass of your ship (including everything, with the engine).  Multiply that by 9.81 m/s2 (Kerbin gravity).  Multiply that by your desired TWR (for example, 1.5 is a good number).  That's how much thrust you need from your engine, in kN.  If the engine you picked has approximately that much thrust, or a bit more, then it's good.  If the engine you picked has a lot less than that, you need a more powerful engine.  If the engine you picked has much more thrust than that, you have too much engine-- find something smaller.

The other gotcha to watch out for is that some engines are "vacuum engines" that get very poor performance in air (they're intended for space), so don't use them on the launchpad, they're for upper stages.  These engines include the Terrier, the Poodle, and the Rhino.

So, suppose you have a ship with a Reliant engine on it, and a total mass of 16 tons (including the engine).  16 tons, times 9.81 m/s2, makes your ship weight about 157 kN.  The Reliant gets 200.67 kN of thrust at sea level.  So your TWR would be 200.67 / 157 = 1.28 on the pad.  That's a little bit on the low side, but it's doable.  So a Reliant engine would be a reasonable choice for that ship.

Anyway, that should give you the general idea.

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If it's going to do lots of transfers between bodies in space I'd think about going with a Nerv engine.  It's a bit slow but it has the range to go farther then any other engine, except for ion.  Otherwise, as Sir_Robert said, the Terrier is a great engine for that type of job.

 

:edit: Also, if you go the Nerv route, your going to need radiators to cool the engine down.  4 large surface mount radiator panels will do the job nicely.  Or a single deployable medium radiator array will work nearly as well, and can be turned off once the craft has cooled down.

Edited by Jakalth
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30 minutes ago, JWOC said:

the capsule and service module is 15.4 tons by itself. i plan on using it to do missions in low to high kerbin orbit, duna and the moon. and other planets.  it will strictly fly in space. @geschosskopf thanks. 

So, if your question is just "I've already gotten it into Kerbin orbit, what do I use as my main engine in space?" then a Terrier will fit the bill pretty well, as Sir_Robert suggests.

If, on the other hand, your problem is "how do I get this thing into space from the launchpad?" then that's a very different question, would be helpful to see a screenshot.

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If the craft is going to fly strictly in space and not land on anything, two to four Spark engines will do nicely. I've also found that a single Poodle works well; it's heavier, but has better fuel efficiency and gets those big transit burns done a lot quicker.

For top fuel efficiency at the expense of taking longer to do a burn, consider a single nuclear engine. The fuel efficiency is more than twice as good as anything else except an ion engine, so you're going from a 15.4 ton ship to 18.4 tons to get twice the burn time, which is a good tradeoff. The only real problem with a nuclear engine is, its length makes it tough to fit on a small spacecraft; you'll want to use the Offset tool to shove the engine up into the main hull, and MAKE SURE YOU REMOVE THE FAIRINGS! Otherwise when you activate the engine the fairings will detach INSIDE your ship and the random collisions will cause an occasional R.U.D. incident.......

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I'd use the terrier, if it wasn't for the Duna bit. While Duna barely has an atmosphere... it does have one. So I'd pick the aerospike. I'd need to try it in the VAB, but I'm under the impression 15.4 tons is too light for Nervs, specially if it's a Duna lander (would it have the TWR to take off?)

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35 minutes ago, juanml82 said:

I'd use the terrier, if it wasn't for the Duna bit. While Duna barely has an atmosphere... it does have one. So I'd pick the aerospike. I'd need to try it in the VAB, but I'm under the impression 15.4 tons is too light for Nervs, specially if it's a Duna lander (would it have the TWR to take off?)

Yup, you can use a single LV-N for a Duna lander, here's one I made for many of my upcoming planetary missions:

Ii3Zg44.png

The atmo TWR is lower, but still plenty to take off.  The tanks are LF ones from Fuel Tanks Plus, but stock ones would work too.

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10 hours ago, JWOC said:

the capsule and service module is 15.4 tons by itself. i plan on using it to do missions in low to high kerbin orbit, duna and the moon. and other planets.  it will strictly fly in space. @geschosskopf thanks. 

Sounds like you are talking about a transfer or tug stage, something to get your payload from one planet's orbit to another? In which case, the general advice is to use an engine(s) with high vacuum Isp (kinda like MPG) and the lowest mass and so thrust you can live with (lower TWR means longer burns but in space gets you there just the same as high TWR). That way you will be using an efficient engine with low mass.   

I've knocked together a craft with a payload like yours...

fFZVRja.png

This transfer stage with one nuke and three LF tanks gives 2135dV. 

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7 hours ago, GeneralVeers said:

If the craft is going to fly strictly in space and not land on anything, two to four Spark engines will do nicely. I've also found that a single Poodle works well; it's heavier, but has better fuel efficiency and gets those big transit burns done a lot quicker.

For top fuel efficiency at the expense of taking longer to do a burn, consider a single nuclear engine. The fuel efficiency is more than twice as good as anything else except an ion engine, so you're going from a 15.4 ton ship to 18.4 tons to get twice the burn time, which is a good tradeoff. The only real problem with a nuclear engine is, its length makes it tough to fit on a small spacecraft; you'll want to use the Offset tool to shove the engine up into the main hull, and MAKE SURE YOU REMOVE THE FAIRINGS! Otherwise when you activate the engine the fairings will detach INSIDE your ship and the random collisions will cause an occasional R.U.D. incident.......

Actually Sparks are less efficient than Terriers

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16 hours ago, Snark said:

itySo, if your question is just "I've already gotten it into Kerbin orbit, what do I use as my main engine in space?" then a Terrier will fit the bill pretty well, as Sir_Robert suggests.

If, on the other hand, your problem is "how do I get this thing into space from the launchpad?" then that's a very different question, would be helpful to see a screenshot.

what i am asking is "I've already gotten it into Kerbin orbit, what do I use as my main engine in space?" @Snark sry dont have pics atm i'm currently using my pc. my laptop has the craft file on it.  i can tell you what the craft is though. it is launches om  Thunder 1B launch vehicle by @Raptor9, with another orange tank added to it, 2 of the large srbs. bigger fins and a 2.5 to 3.5 adapter to interface with the fairing,. the spacecraft is @Raptor9's EV-2A modified to have a Rockomax Brand Adapter 02 on the bottom with 4 Stratus-V Roundified Monopropellant Tanks gizomoed into it. it also has an omnidirectional antenna attached to the front of the service bay. i also had another questions as snark has already mentioned. it is how to get into space from the launchpad. currently i have a terrier on it. and i was wondering if 2 ks-25s would be a better option . i also have another question when using 2 ks-25s and i am high up in the atmosphere. it likes to tip over, and i do use mechjeb. 

Edited by JWOC
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3 minutes ago, JWOC said:

what i am asking is "I've already gotten it into Kerbin orbit, what do I use as my main engine in space?"

On short term jobs, Terrier all the way. On very long term jobs where you aim to refuel a ship multiple times, go LV Nerv.

If you want more thrust, a cluster of these engines

Edited by Sir_Robert
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11 hours ago, juanml82 said:

I'd use the terrier, if it wasn't for the Duna bit. While Duna barely has an atmosphere... it does have one. So I'd pick the aerospike. I'd need to try it in the VAB, but I'm under the impression 15.4 tons is too light for Nervs, specially if it's a Duna lander (would it have the TWR to take off?)

Terriers work just great on Duna.

Yes, the Terrier is a vacuum engine... but "sea level" on Duna is over 93% vacuum.  Maybe this means that the Terrier's Isp will drop from its 345 vacuum value down to 330 or something, but that still makes it more efficient than just about anything else.  And above "sea level", the atmosphere drops off very rapidly.

Heck, the Terrier works pretty well even in Kerbin's atmosphere, as long as you're above 10 km or so altitude.

Regarding LV-N's:  They're a great workhorse rocket for orbital transfer, but a terrible idea for landers unless it's somewhere with really low gravity.  The engine is very heavy, so the benefit goes up with larger ship mass; 15.4 tons is kinda borderline, but probably still worthwhile.

One thing I'd advise:  in general, spacecraft that are specially designed for one purpose do a lot better than spacecraft that try to do multiple things.  If you build a spacecraft that's optimized for interplanetary transfers, it won't be a good lander, and vice versa.  So if this is a "workhorse" vehicle that's intended for long usage over multiple missions, I would suggest not trying to be both a transfer vehicle and a Duna lander.  Have one vehicle that's a dedicated Duna lander that shuttles back and forth between Duna surface and Duna orbit, and a different vehicle which is for interplanetary transfer, which goes between orbit of planet A and orbit of planet B.

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12 hours ago, GeneralVeers said:

If the craft is going to fly strictly in space and not land on anything, two to four Spark engines will do nicely.

 

5 hours ago, Sir_Robert said:

Actually Sparks are less efficient than Terriers

+1 to this.  I love Sparks, they're neat little engines, but their Isp is really not very good; the Terrier is much better for that.

The main use for a Spark is in a very tiny, lightweight craft where the extra mass of the Terrier would outweigh the benefit from added fuel efficiency.  Terriers get 15% more bang for the buck, at the cost of 400kg added mass, so basically the Terrier's a win if those added 400kg represent less than 15% of the ship mass.  So for ship that's 15 tons or more, the Terrier would be a clear win over a Spark.

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15 minutes ago, Snark said:

Terriers work just great on Duna.

Yes, the Terrier is a vacuum engine... but "sea level" on Duna is over 93% vacuum.  Maybe this means that the Terrier's Isp will drop from its 345 vacuum value down to 330 or something, but that still makes it more efficient than just about anything else.  And above "sea level", the atmosphere drops off very rapidly.

Heck, the Terrier works pretty well even in Kerbin's atmosphere, as long as you're above 10 km or so altitude.

Regarding LV-N's:  They're a great workhorse rocket for orbital transfer, but a terrible idea for landers unless it's somewhere with really low gravity.  The engine is very heavy, so the benefit goes up with larger ship mass; 15.4 tons is kinda borderline, but probably still worthwhile.

One thing I'd advise:  in general, spacecraft that are specially designed for one purpose do a lot better than spacecraft that try to do multiple things.  If you build a spacecraft that's optimized for interplanetary transfers, it won't be a good lander, and vice versa.  So if this is a "workhorse" vehicle that's intended for long usage over multiple missions, I would suggest not trying to be both a transfer vehicle and a Duna lander.  Have one vehicle that's a dedicated Duna lander that shuttles back and forth between Duna surface and Duna orbit, and a different vehicle which is for interplanetary transfer, which goes between orbit of planet A and orbit of planet B.

its not going to be a lander. iit will just orbit the planet 

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51 minutes ago, juanml82 said:

Nukes then. For anything that won't land and goes beyond Kerbin's SOI, nukes are the best.

Here's a surprise for ya: my first-ever Duna lander used dual nuclear engines! Duna's atmosphere is thin enough that the ISP of nuclear enignes isn't significantly affected.

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1 hour ago, juanml82 said:

Nukes then. For anything that won't land and goes beyond Kerbin's SOI, nukes are the best.

ONLY for ships that are heavy. If your engine makes up 50% of your ships mass, you are doing it wrong. Contrary to popular belief, nukes are definitely not always the best

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1 hour ago, juanml82 said:

Nukes then. For anything that won't land and goes beyond Kerbin's SOI, nukes are the best.

...with the caveat that the above statement is only true for fairly massive ships, and becomes more so as ship mass increases.  For small ships, the extra mass of the LV-N will more than offset its improved fuel efficiency.  But yeah, for 15 tons and up, the nuke is a win.

34 minutes ago, GeneralVeers said:

Here's a surprise for ya: my first-ever Duna lander used dual nuclear engines! Duna's atmosphere is thin enough that the ISP of nuclear enignes isn't significantly affected.

Well, sure, you can do it, but that doesn't mean it's necessarily a good idea.  :)

Yes, nukes work just fine in Duna's atmosphere.  However, the issue is one of TWR.  Duna has fairly significant gravity, 0.3g at the surface.  Nukes are massive and have very poor TWR, which doesn't matter in orbit but really matters in a lander.

This means that a nuke-powered lander on Duna is going to pile up substantial gravity losses on takeoff, much more than a lighter, more powerful engine would.  The only way to offset that would be to try to keep the ship mass fairly light so that you can get a good-enough TWR even with the nukes aboard... which means that now you're paying a substatially bigger penalty for the added mass of the nukes.

For example, let's say you want an acceleration on Duna takeoff of 10 m/s2 (i.e. about one Kerbin gravity), to keep gravity losses manageable.  With two nukes at 60 kN each (actually just a smidgeon less due to Duna's atmosphere, but let's ignore that), that puts an upper limit on your takeoff mass of 12 tons.  So... you've got two nukes on a 12-ton lander?  That's half of your total mass that you just spent on engines!  Leaving not a lot of room for fuel, the command pod, whatever other paraphernalia you have along for the ride.

Whereas, if you had used Terriers instead-- that is, keep the exact same design, but swap out Terriers for LV-N's-- then you would shave off 5 tons of dead weight, which ought to be more than enough to make up for the lowered fuel efficiency.

And if the ship is less massive... the penalty for the extra mass of the LV-N's just goes up.

And if the ship is more massive... then you get hammered with gravity losses, which would be a lot lower (and therefore save fuel) with lighter, more powerful engines.

...So what it boils down to is, yeah, you can certainly build a nuke-powered Duna lander.  And if you're doing it just for the grins, 10 out of 10 points for style.  :)  But from the standpoint of engineering efficiency, it's not a great choice.

(It would have been a significantly less-bad choice pre-1.0 than it is now, because LV-N's were fully 750 kg lighter then.  Not sure when you did that Duna landing.)

Edited by Snark
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Pretty long post. ^_^ The design considerations took a lot less words than that: my Duna lander was around 12 tons before the engines went on. So it was either 1-2 more tons for standard engines, or 4-6 more tons (I forget which version I did the landing in) for nukes at just under twice the ISP. Either way, the nukes were the best choice.

When you "get hammered with gravity losses", it's not the weight of the engines that counts. It's the total weight of the entire vehicle. If you get twice the ISP and you less than double the weight of the vehicle, you made a good tradeoff.

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15 minutes ago, GeneralVeers said:

Pretty long post. ^_^ The design considerations took a lot less words than that: my Duna lander was around 12 tons before the engines went on. So it was either 1-2 more tons for standard engines, or 4-6 more tons (I forget which version I did the landing in) for nukes at just under twice the ISP. Either way, the nukes were the best choice.

When you "get hammered with gravity losses", it's not the weight of the engines that counts. It's the total weight of the entire vehicle. If you get twice the ISP and you less than double the weight of the vehicle, you made a good tradeoff.

You are also ignoring the amount of fuel it costs to get those Nuke engines out to Duna in the first place. That would have been significantly less with Terriers

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